1. Field of the Invention
The present invention relates to an image projection system implemented with a mirror device manufactured by the field of Micro Electro Mechanical Systems (MEMS) technology. More particularly, this invention relates an image projection system wherein the optical axis of the illumination light incident to the mirror device and the optical axis of the reflection light in the projecting direction forms an angle that is larger than the expansion angle α of the flux of the illumination light that satisfies NA=n*sin α, where NA is the numerical aperture of the flux of the illumination light, and n is the reflectance.
2. Description of the Related Art
After CRT technology dominated the display industry for over 100 years, Flat Panel Display (hereafter FPD) and Projection Display attained popularity because of the improved form-factor and a larger size screen. In several types of projection displays, projection displays using micro-displays are gaining recognition by consumers because of high performance image quality and lower cost advantage compared to the FPDs. There are two types of micro-displays used for projection displays in the market: One is a micro-LCD (Liquid Crystal Display), and the other is a display using mirror technology such as a mirror device. Since the mirror device uses un-polarized light, the mirror device has an advantage in brightness over the polarized light using micro-LCD.
Even though there are significant advances in the technologies implementing an electromechanical mirror device as a spatial light modulator (SLM) in recent years, there are still limitations and difficulties when a spatial light modulator is employed to provide a high quality image. Specifically, when the images are digitally controlled, the image quality is adversely affected due to the fact that the images are not displayed with a sufficient number of gray scales.
Electromechanical mirror devices are drawing considerable interest as the spatial light modulators (SLM). The electromechanical mirror device includes “a mirror array” arranging a large number of mirror elements. In general, the mirror elements, ranging in number from 60,000 to several millions, are arranged on the surface of a substrate in an electromechanical mirror device.
The mirror device includes multiple mirror elements to function as a spatial light modulator (SLM). Each mirror element includes a mirror and electrodes. A voltage applied to the electrode(s) generates a coulomb force between the mirror and the electrode(s), thereby making it possible to control the incline of the mirror and the mirror is “deflected,” according to a common term used in this specification for describing the operational condition of a mirror element.
When a mirror is deflected with a voltage applied to the electrode(s) to control the mirror, the deflected mirror also changes the direction of the reflected light in reflecting an incident light. The direction of the reflected light is changed in accordance with the deflection angle of the mirror. The present specification refers to a state of the mirror when incident light is reflected towards a projection path designated for image display as an “ON light”, while referring to a light reflected in a direction other than the designated projection path for image display as an “OFF light”.
One of the spatial light modulators known as a Digital Mirror Device (DMD) is formed and supported on a device substrate. The DMD includes a plurality of mirror elements for modulating incident light emitted from a light source and for controlling the reflecting direction of the incident light by deflecting mirrors for projecting an images through an image projection system. A projection apparatus often employs a discharge lamp light source, such as a mercury lamp or a xenon lamp. The image projection system also employs a DMD to control the reflecting direction of light with the mirrors of the DMD, so that an ON light incident to a projection lens does not overlap with an OFF light not incident to the projection lens. Thereby, the contrast of a projection image is enhanced.
For example, a specific reference document, i.e., U.S. Pat. No. 6,885,494, discloses that degradation in the contrast and the image quality of an image is caused by the ON light and OFF light, which are reflected by a mirror, overlapping with each other. Further, the document discloses a technique for improving the aforementioned problem by setting the difference in deflection angles between the ON light and OFF light states at 28 degrees.
Meanwhile, a projection apparatus using a DMD disclosed by USA-based Texas Instruments, Inc. is configured to set the deflection angles of the mirror of the DMD at ±12 clockwise (CW) relative to the initial state of the mirror, corresponding to the ON and OFF lights respectively. Such a configuration separates a projection light path of the ON light from an OFF light path directing toward a light shield member. The configuration thus provides separate light paths without causing the f/2.4 illumination light flux emitted from a light source modulated as the ON light and OFF light to overlap with one another. Further, the apparatus is configured to place mirror elements so as to prevent the reflection light or diffraction light generated by a mirror in the initial state from heading towards the projection light path. As such, the deflection angle of the mirror of the conventional DMD is designed to prevent light fluxes, which are determined by the F-number of a projection lens so as to obtain a sufficient level of resolution by effectively utilizing the light output from a discharge lamp light source, from overlapping with one another.
An image projection apparatus may also use a light source with a coherent characteristic such as a laser light source.
The laser light from a laser light source has a coherent characteristic because the laser light has a uniform wavelength and phase. The laser light therefore can project with a high directivity and maintains continuously along the optical path as parallel light beam. More recently, there has been much progress in the development of a compact, high-output laser light source.
A more compact optical system, compared to that of a projection apparatus comprising a discharge lamp and a spatial light modulator with a smaller deflection angle of mirror, is desired in order to display an image with high resolution.
The utilization of a laser light source for a projection apparatus, however, introduces a diffraction phenomenon due to the high degree of coherence. As further explained below, this phenomenon-affects image quality.
Particularly, a spatial light modulator, such as a DMD includes mirror elements arranged as mirror element arrays in constant intervals functioning similar to a diffraction grating. A diffraction phenomenon occurs when the light is reflected by the spatial light modulator in a projection apparatus with a combination the spatial light modulator and the coherent light source. As a result, such diffraction light is projected to a projection light path thus causes the problem of inadvertently increasing the overall brightness of the images. For this reason, there is a technical issue concerning a device configuration to prevent diffraction light from entering the projection light path.
There are many additional technical problems related to the application of the DMD in an image projection system, such as diffraction of light caused by a through hole for mounting a mirror onto a post, the reflection of light from the gap the between adjacent mirrors and by the edges of a mirror, and diffusion of the OFF light that may interfere with the image projection light. Consequently, it is important to properly configure an image projection system to prevent such extraneous light from entering a projection optical system in order to improve image quality, by enhancing the contrast of a projection image.